1. Technical Field
The invention relates to a line parameter capacitance estimation method and system and a fault management system incorporating such a system.
2. Related Art
A conventional public telecommunications network comprises a relatively small number of interconnected main switches and a larger number of local switches, each of which is connected to one or two main switches. The local switches are connected to the terminating lines of the network and the far ends of these lines are connected to terminal equipment such as telephone instruments provided for users of the network.
The network formed from the main switches and local switches is known as the core network, while the network formed from the terminating lines is known variously as an access network or a local loop. Some terminating lines are connected to a remote concentrator, which may or may not have switching capabilities. The remote concentrator is then connected to a local switch. The term local switch when used herein covers both local switches and remote concentrators.
In a conventional access network, each terminating line is formed from a pair of copper wires. Typically, each copper wire passes through a series of nodes between the local switch and terminal equipment. Examples of such nodes are primary cross-connection points (PCP), secondary cross-connection points (SCP), distribution points (DP) and junctions.
Terminating lines are prone to faults such as dis-connections, short circuits between the two wires of the pair of wires, and short circuits between one of the wires and earth. Causes of such faults include ingress of water into a node and also physical damage to a node.
Disconnection faults occur frequently; 30-40% of all logged faults are disconnection faults and 25% are at the PCP making these the most common kind of disconnection fault. According to known systems, the engineer is sent to the PCP to establish whether the fault is there or in the vicinity which may be as little as a street or two away. However it is desired to be able to identify within a reasonable level of accuracy whether the fault is actually at the PCP or, for example, with the customer, in order to reduce the engineer time spent on locating the fault.
Local switches are provided with line testing apparatus which may be used to test its terminating lines. When a customer reports a fault on a terminating line, the line may then be tested to identify the fault condition.
The process of locating, evaluating and repairing a fault may involve a comparison between the fault value with a reference capacitance which is the capacitance of a line under normal conditions between a node, such as a PCP, and the local switch.
Conventionally, it has been necessary for a person to travel to a node to test the node. Simultaneously, it is necessary to test the line from the local switch. This procedure suffers from several disadvantages. Firstly, as it is necessary to send a person to the node to measure a capacitance reference value, two people are needed to complete the measurements—one at the node and a person at the local switch. The procedure itself is labour intensive, and time consuming. Also, because it is also necessary to test several nodes before the correct one is found, the problems of cost and time are exacerbated. Known systems are described in published patent applications EP 0862828 and EP0938800.
According to one known system, the location of the fault is then estimated using an expert system assigning scores based on a range of parameters. This system requires complex processing of data on site. In addition, where there is more than one line running to the PCP, or cabinet, a single reference value may not be representative if there is re-routing or if a line between the switch and exchange follows an alternative route where there are multiple lines.